A large body of literature on health effects of relative humidity in indoor environments suggests that relative humidity can affect the incidence of respiratory infections and allergies. Experimental studies on airborne-transmitted infectious bacteria and viruses have shown that the survival or infectivity of these organisms is minimized by exposure to relative humidity between 40 and 70%.

Nine epidemiological studies examined the relationship between the number of respiratory infections or absenteeism and the relative humidity of the office, residence, or school. The incidence of absenteeism or respiratory infections was found to be lower among people working or living in environments with mid-range versus low or high relative humidity.

Most species of fungi cannot grow unless the relative humidity exceeds 60%. Relative humidity also affects the rate of off gassing of formaldehyde from indoor building materials, the rate of formation of acids and salts from Sulphur and nitrogen dioxide, and the rate of ozone formation.

The influence of relative humidity on the abundance of allergens, pathogens, and noxious chemicals suggests that indoor relative humidity levels should be considered as a factor of indoor air quality.

Conclusion

The majority of adverse health effects of low relative humidity can be minimized by maintaining indoor levels between 40 and 60%. This requires humidification during winter in areas with cold winter climates.

Humidification should preferably use evaporative or steam humidifiers, as cool mist humidifiers can disseminate aerosols contaminated with allergens, bacteria and fungi.

Arundel’s review is the most important piece of evidence underpinning the accuracy of the decision, to keep humidity in the range of 40 to 60%.

The review itself and the 99 references provide a wealth of conclusive evidence for keeping humidity in the midrange and the only one that includes findings from interventional studies with health outcomes. The presented scientific findings from 1986 are very close to today’s level of knowledge and include facts, we have forgotten or suppressed in the meantime.

Arundel and his co-authors (all engineers!) are very clear in their statement that optimal humidity in heated buildings cannot be achieved without active humidification. They knew, that the best way of achieving this goal under hygienic conditions is evaporative or steam humidification.